The present invention relates generally to semiconductor processing and, more particularly, to a system and method for determining the resolution of an electron beam in a scanning electron microscope by measuring and/or imaging features, such as lines and spaces, having a re-entrant profile.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers (e.g., at submicron levels). In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, x-rays, etc.) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Due to the extremely fine patterns which are exposed on the photoresist, Scanning Electron Microscopes (SEMs) often are employed to analyze and measure critical dimensions resulting from the lithographic process. Critical dimensions include the size of minimum features across the wafer such as linewidth, spacing, and contact dimensions. Although SEMs have been effective, they are limited by the resolution or shape of an electron beam employed in the imaging process. The electron beam shape becomes elliptical over time due to electronic degassing from the wafer or sample being employed. Degradation or widening of the beam width in the x and y direction influences adversely the resolution of the SEM device. The wider the beam the more difficult it is to image accurately smaller features. Essentially, a beam that is not circular will have feature reading with resolutions that differ when different portions of the beam cross the feature.
In certain fabrication processes, resist and/or etched features have cross-sectional profiles that are re-entrant. By xe2x80x9cre-entrant profile,xe2x80x9d it is meant that the sidewalls of the feature taper inwardly at the bottom of the feature. For an elongated feature, such as a line or space, a re-entrant profile may result in an elongated trench (e.g., having a triangular cross section) positioned along the juncture of the feature and the substrate surface parallel to the substrate surface. Re-entrant profile features have images with better resolution because side edges of the features being measured do not interfere with the measurement.
There is an unmet need to have a system and/or method which is able to determine and control the resolution of a SEM to facilitate improved measuring and/or imaging of a feature, such as a line and/or space. There is also an unmet need to have a system and/or method which is able to determine and control the shape of an electron beam employed by a SEM.
The present invention relates to a system and method for measuring and determining the resolution of a SEM imaging system employing an etched sample with a re-entrant cross-sectional profile. A re-entrant or negative profile is employed because the top-down view seen by the SEM is very sharp due to the fact the edge of the profile has zero width. Therefore, any apparent beamwidth seen in the signal is a function of the electron beam width alone. Scanning the beam across the profile provides a signal that moves from a first state to a second state. The time period or sloping portion of the signal from the first state to the second state (hereinafter referred to as the intermediary state) provides a correlation to the electron beam width. Thus, scanning across the sample allows for a calculation of the electron beam width. By scanning across features having different orientations, the shape of the electron beam can be determined. Alternatively, by rotating the electron beam and scanning across the same feature, the shape of the electron beam can be determined. A system can utilize this information to adjust the roundness of the electron beam. Alternatively, the system can utilize this information to adjust the resolution of the SEM or a display displaying the image.
One aspect of the present invention relates to a method for determining the resolution of a measurement system employing an electron beam and a detector for detecting secondary emissions and generating a signal corresponding to the secondary emissions. The method comprises the steps of providing a wafer with at least one reentrant feature and performing a scan of the at least one re-entrant feature with the electron beam to provide a signal corresponding to an image of the re-entrant feature. The signal has a first state, an intermediary state and a second state. The intermediary state of the signal is the evaluated and a width of the electron beam is determined based on the evaluation.
Another aspect of the invention relates to a method for determining the shape of an electron beam in a SEM system. The method comprises the steps of scanning a first re-entrant feature to produce a first signal and scanning a second re-entrant feature producing a second signal. The second re-entrant feature has a different orientation than the first re-entrant feature. The shape of the electron beam is determined based on evaluation of the first and second signals.
Yet another aspect of the invention relates to a method for determining the shape of an electron beam in a SEM system. The method comprises the steps of scanning a re-entrant feature to produce a first signal, rotating one of the electron beam and the re-entrant feature an angular amount and scanning the re-entrant feature producing a second signal. The shape of the electron beam is determined based on evaluation of the first and second signals.
Another aspect of the present invention relates to a system for determining the resolution of a CD system. The system comprises a measurement system configured for emitting a scanning signal along a substrate having at least one re-entrant feature. The measurement system is configured to produce a first signal based on a first image of the at least one re-entrant feature and determine a width of the scanning signal based on a portion of the first signal.
Yet another aspect of the present invention relates to a system for determining the resolution of a CD system. An emitter directs a beam onto a substrate having at least one re-entrant feature. A movement system is provided that is adapted to move the electron beam across the at least one re-entrant feature. A detector detects interactions between the beam and the at least one re-entrant feature and/or substrate and provides a detector signal indicative of the at least one re-entrant feature. A controller determines a beam width based on the detector signal.
Yet another aspect of the present invention relates to a CD-SEM system primarily for measuring a cross-sectional dimension of a feature having a re-entrant profile relative to a substrate. A lens is provided for directing an electron beam to the surface of the substrate during a first scanning interval and a second scanning interval. A first outer circumferential portion of the electron beam scans across the surface of the substrate during the first scanning interval and a second outer circumferential portion of the electron beam scans across the surface of the substrate during the second scanning interval. A detector provides a signal based upon electrons received from the surface of the wafer. A processing system determines a first electron beam width based on detected electrons associated with the first scanning interval and a second electron beam width based on detected electrons associated with the second scanning interval. The processing system determines a shape of the electron beam based on an aggregation of the first and second electron beam widths.
Yet another aspect of the present invention relates to a system for determining the width of an electron beam. The system comprises means for directing an electron beam signal across a surface of a wafer having at least one re-entrant profile, means for measuring a secondary signal generated as a result of the electron beam signal, means for generating an image signal based on the secondary signal and means for determining a width of the electron beam based on the image signal.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.